In the production of oil or gas from certain types of formations, sand also may be produced and, if the flow of sand from the well is not controlled in some manner, apparatus within the well will almost certainly be damaged by abrasion of the sand. Gravel packing is one method of sand control in which particularly sized particles of a suitable material such as gravel or glass beads or the like, all referred to herein as gravel, are used to filter sand from the produced fluid before flowing into a production string in the well. Generally speaking, gravel packing is accomplished by pumping a liquid slurry into the well with gravel from the slurry being collected at least in part within the casing between the inside wall thereof and a sand screen liner so as to filter sand from the fluid before the latter enters the production string.
One prior method and apparatus for gravel packing a well to prepare the well for production is illustrated schematically and described briefly in Guiberson 1979/80 General Catalog at pages 64 and 65. Briefly, as shown in that catalog, a tubing string carrying a packer is run into a well to position a sand screen liner carried by the packer adjacent the formation to be produced. For running in the well, the packer is latched to the tubing string by way of a slurry tool which is connected to the lower end of the tubing string. The tool includes a latch tube for connection to the packer and a wash pipe telescoped through the latch tube. The wash pipe protrudes from the lower end of the packer into the sand screen liner which includes upper and lower sand screen sections separated from each other by an annular seal. The latter engages the lower end portion of the wash pipe to seal against fluid communication through the interior of the liner between the upper and lower sand screen sections.
With the foregoing arrangement, gravel packing is accomplished after setting the packer by pumping a gravel slurry down the tubing string, into an upper section of the wash pipe, past the packer, and through a crossover valve into the annulus between the well casing and the sand screen liner. The gravel in the slurry is collected at the bottom of the well with the liquid being filtered from the gravel by being driven through the lower sand screen section and into the open lower end of a lower section of the wash pipe. This return liquid flows upwardly through the lower section of the wash pipe, past the crossover valve, through the annular area between the latching tube and the upper section of the wash pipe, and exits the annular area through a circulation valve flowing into an upper well annulus located between the tubing string and the casing above the packer. Thereafter, the return liquid flows toward the top of the well and through the upper annulus ultimately to be dumped from the well.
Periodically, during the packing process, the circulation valve may be closed and high pressure applied through the slurry to compact the gravel collected at the bottom of the well and also to drive some of the gravel into the formation through perforations in the casing. Once gravel is packed in the bottom of the well to some desirable level above the upper sand screen, flow through the well may be reversed for liquid to exit the bottom of the wash pipe as the slurry tool is pulled upwardly. This circulates excess slurry out of the tubing string and helps free the lower end of the wash pipe in the event some deposits have collected in the liner. As the wash pipe is pulled free, the liquid from the upper well annulus, enters the annular area between latch tube and the upper section of the wash pipe, flows downwardly past the crossover valve and into the lower section of the wash pipe. The liquid exits the lower end of the wash pipe and, once the end passes above the annular seal between the sand screen sections, the liquid flows upwardly through the crossover valve and into the upper section of the wash pipe then into the tubing string and to the well head, driving out any excess slurry remaining in the tubing string and thereby completing the gravel packing operation except for pulling the cleaned tubing string from the well to remove the slurry tool.